Method of Diffusing a Catalyst for Electrochemical Oxygen Reduction

ABSTRACT

An improved gas diffusion electrode composed of a perovskite-type oxide dispersed in a mixture of carbon black and a hydrophobic binder polymer. An improved catalyst for use in the electrochemical reduction of oxygen comprising a perovskite-type compound having alpha and beta sites, and having a greater molar ratio of cations at the beta site. A particularly good reduction catalyst is a neodymium calcium manganite. An improved method of dispersing the catalysts with carbon in a reaction layer of the electrode improves performance of the electrode and the oxygen reduction process. This is provided by adding carbon black to an aqueous solution of metal salts before it is heated to a gel and then to a char and then calcined. Optionally, a quantity of the desired oxide catalyst can be premixed with a portion the carbon before adding the carbon to an aqueous solution of the metal salts to be heated. The amount of premixed metal oxide is chosen in conjunction with the amount of metal salts to provide the desired molar ratio after heating and calcining of the aqueous solution.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of, and claims priority to, applicationSer. No. 10/708,565, filed Mar. 11, 2004, entitled Gas DiffusionElectrode and Catalyst For Electrochemical Oxygen Reduction and Methodof Dispersing the Catalyst, which application is incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to gas diffusion electrodes, catalysts,and electrolytic cells suitable for chlor-alkali electrolysis, metal-airbatteries, fuel cells and the devices concerned with electrochemicaloxygen reduction, such as oxygen sensors.

2. Background Art

Gas diffusion electrodes are used as oxygen cathodes suitable forchlor-alkali electrolysis, metal-air batteries, and fuel cells.

A gas diffusion electrode has a multilayer structure composed of a gasdiffusion layer, a reaction layer, and a current collector forelectrical connection. Gas phase oxygen is exposed to the gas diffusionlayer. The reaction layer resides between the gas diffusion layer and anelectrolyte. After passing through the gas diffusion layer, oxygen isconsumed through a reduction reaction on an oxygen reduction catalyst inthe reaction layer. The reaction proceeds according to the followingequation:O₂+2H₂O+4e ⁻=4OH⁻

The gas diffusion layer is required to allow the oxygen to passtherethrough rapidly and to diffuse uniformly into the entire reactionlayer. The gas diffusion layer is also required to prevent theelectrolyte from permeating to the gas phase. The gas diffusion layer iscomprised of a material formed of carbon particles bonded to each otherwith a material, such as polytetrafluoroethylene, having high waterrepellent properties. The gas diffusion layer must also conductelectrons from the current collector to the reaction layer.

The reaction layer contains uniformly dispersed oxygen reductioncatalyst particles in electronic continuity with the gas diffusion layerand current collector. In the reaction layer, a large interface area isformed among the oxygen, electrolyte, electrons, and the oxygenreduction catalyst.

The particles for forming the reaction layer can be prepared, forexample, by mixing carbon particles, intermixed with an electrodecatalyst with a dispersion of a fluorinated resin such aspolytetrafluoroethylene, dispersing the mixture thus formed using adispersing agent such as an alcohol, filtrating and drying thedispersion thus formed, and pulverizing the dried material into fineparticles. Methods for processing the gas diffusion electrode layers aredisclosed by Furuya in U.S. Pat. Nos. 6,630,081 and 6,428,722.

The current collector may be, for example, a wire mesh or a foammaterial, which is composed of nickel, silver, or the like.

Mainly noble metals such as platinum and silver, either dispersed in orsupported on carbon black in the reaction layer, have been used andinvestigated as oxygen reduction catalysts for concentrated alkalinesolution.

In the chlor-alkali application, utilization of an oxygen consumingcathode instead of a hydrogen evolving cathode provides the opportunityof reducing the specific power requirement since the theoreticaloperating voltage is lower by approximately 1.23V.

However, the over-voltage of electrochemical oxygen reduction gasdiffusion cathodes using previous catalysts is high or increases overtime. In the case of industrial chlor-alkali electrolysis, if the overvoltage is too high, economical advantages of electrolyzers using gasdiffusion cathodes are not realized compared to conventionalelectrolyzers using hydrogen cathodes. Accordingly, a stable catalystwith high catalytic activity for electrochemical oxygen reduction isneeded to lower the over-voltage. Such a catalyst may also providebenefits for oxygen cathodes used in non chlor-alkali applications.

T. Hyodo et al. has shown Pr_(0.6)Ca_(0.4)CoO₃ perovskites to outperformPt as an oxygen reduction catalyst in alkaline electrolyte, at least for200 hours where air is the oxidant. Similarly, M. Hayashi et al. haveshown La_(0.8)Rb_(0.2)MnO₃ to outperform Pt for 100 hours. Kudo et al.reported that a Nd_(0.8)Sr_(0.2)Co_(1−y)Ni_(y)O₃ catalyst electrode wasstable over 300 hours with a potential >−50 mV relative to a Hg/HgOreference electrode at a presumably low current density and forNd_(0.8)Sr_(0.2)CoO₃ at 100 mA/cm² the potential was −150 mV, and −250mV for Nd_(0.8)Ca_(0.4)CoO₃. G. Karlsson demonstrated −200 mV at 150mA/cm² with Nd_(0.2)Ca_(0.8)MnO₃. Yuasa et al. reported approximately−60 and −65 mV relative to a Hg/HgO reference electrode at 400 mA/cm²current density for a very short test of 3 hours in duration forLa_(0.8)Sr_(0.2)MnO₃ and La_(0.8)Sr_(0.2)Mn_(0.8)Fe_(0.2)O₃. T. Hyodo etal. also claimed Fe in the perovskite composition aided stability whenrunning La_(0.5)Sr_(0.5)FeO₃ approximately −225 mV relative to Hg/HgOreference electrode at 325 mA/cm² current density for 90 hours.

Thus many perovskite-type oxides have been shown to have high catalyticactivity, where the over-voltage of gas diffusion electrodes using thesecatalysts was lower than that of gas diffusion electrodes using platinumor silver. However, neither the stability of perovskite-type oxides in aconcentrated alkaline solution nor the long-range durability of gasdiffusion electrodes using perovskite-type oxides have previously beenconfirmed, especially at high current density.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide an electrocatalystmaterial to be used in an oxygen gas diffusion cathode, able to provideover-voltage lower than precious metals at high current density and fora long duration in an alkaline electrolyte.

Another objective is to provide a method for dispersing theelectrocatalyst with the carbon black used in the gas diffusion reactionlayer. According to one aspect of the invention a catalyst for use inthe reduction of oxygen comprises a perovskite-type compound havingalpha and beta sites, and having a greater molar ratio of cations at abeta site of the compound.

According to another aspect of the invention a preferred catalyst hasthe formula A_(1−x)A′_(x)(B_(1−y)B′_(y))_(z)O_(3+δ) where x, y and z aremole fractions and z has a value greater than one.

According to another aspect of the invention a catalyst according to theabove formula is made where x has a value in the range of 0.01<X<0.9; yhas a value in the range of 0≦Y≦0.9; δ has a value in the range of−0.3<δ<0.3; and A is a metal selected from the group consisting of La,Pr and Nd; A′ is one or more metals selected from the group consistingof K, Rb, Cs, Ca, Sr, and Ba; these metals are said to be on the “Asite” or “alpha site”; B is a metal selected from the group consistingof Mn, and Co; and, B′ is one or more metals selected from the groupconsisting of Fe, and Ni; these metals are said to be on the “B site” or“beta site”.

According to another aspect of the invention,Nd_(1−x)Ca_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ) is proposed as a materialparticularly stable and active as a catalyst for the gas diffusionelectrode. In a preferred embodiment, x has a value in the range of0.1≦X≦0.6, y has a value in the range of 0≦Y≦0.4; δ has a value in therange of −0.3<δ<0.3; and z has a value in the range of about 0.95 toabout 1.05.

According to another aspect of the invention a perovskite catalyst maybe formed such that cations at the beta site have a smaller crystalionic radius than the cations at the alpha site.

According to one aspect of the invention, a method is provided whichparticularly suited for obtaining excellent dispersion of the catalystin an electrode with the carbon used in electrode fabrication. In thismethod a portion or none of the desired oxide is premixed with thecarbon black to ultimately be used for electrode fabrication, and thenthe balance of the desired oxide is prepared by adding an aqueoussolution containing metal salts at a predetermined molar ratio alongwith a dispersant. The slurry is dried then calcined in a non-oxidizingenvironment.

Other aspects and advantages of the invention will be understood bythose of skill in the art by further reference to the following Figures,description of exemplary embodiments, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a gas diffusion electrodeaccording to the invention;

FIG. 2 is a graph showing the results of X-Ray diffraction of catalystcomposition Nd_(0.6)Ca_(0.4)(Mn_(0.8)Fe_(0.2))_(1.01)O_(3+δ) immersed inan aqueous NaOH solution at a concentration of 33 percent by weigh;

FIG. 3 is a graph showing cathode potential versus current density forgas diffusion electrodes made in accordance with the Examples disclosedherein;

FIG. 4 is a graph showing cathode over-potential versus current densityfor gas diffusion electrodes made according to Examples disclosedherein;

FIG. 5 shows diffractograms related to the gas diffusion electrodes madein accordance with the invention with counts per second on the verticalaxis and 2 theta on the horizontal axis;

FIG. 6 is a graph showing cathode potential versus current density forgas diffusion electrodes made in accordance with the Examples disclosedherein;

FIG. 7 is a graph showing cathode over-potential versus current densityfor gas diffusion electrodes made in accordance with Examples disclosedherein;

FIG. 8 is a graph showing a plot of the polarization curves of Examples1, 2 and 3 disclosed herein, against data reported in literature forcathodes made from other materials; and

FIG. 9 is a graph showing polarization curves indicating the effects ofcompounds formulated to be beta site rich according to the Examplesdisclosed herein.

BEST MODE FOR PRACTICING THE INVENTION

The following is a description of preferred embodiments of theinventions. The drawings, the examples, and discussion relative to theembodiments are for exemplifying and illustrating aspects of theinvention and are not intended to limit the claims to any embodimentdisclosed.

Catalyst

A catalyst according to the invention that is particularly useful for agas diffusion electrode is a stable and highly active perovskite oxiderepresented by the formula A_(1−x)A′_(x)(B_(1−y)B′_(y))_(z)O_(3+δ)wherein A is a metal selected from the group consisting of La, Pr andNd; A′ is one or more metals selected from the group consisting of K,Rb, Cs, Ca, Sr, and Ba; B is a metal selected from the group consistingof Mn, and Co; B′ is one or more metals selected from the groupconsisting of Fe, and Ni; X has a value in the range of 0.01<X<0.9; Yhas a value in the range of 0≦Y≦0.9; δ has a value in the range of−0.3<δ<0.3; and, where z has a value in the range 0.95<z<1.05.

Whether a cation is an A or A′ versus a B or B′ cation is determinedlargely by its crystal ionic radius where substantially, cations in therange 1.0-1.7 Å are stable on the “A” site and cations in the range0.59-0.80 Å are stable on the “B” site. In a nominal perovskitecomposition, Z=1 where the “A” site cations are about equal to the “B”site cations, but it is possible to vary the perovskite propertiessomewhat by varying Z. If Z<1.0 then the composition is “A” site rich.If Z>1.0 then the composition is “B” site rich.

According to another aspect of the invention, all the perovskite-typecatalysts are more suitable for an oxygen reduction catalyst whensynthesized to be “B” site rich; that is when Z>1.

La_(1−x)Rb_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ),La_(1−x)Cs_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ),Pr_(1−x)Sr_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ),Pr_(1−x)Ca_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ),Nd_(1−x)Ca_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ),Nd_(1−x)Sr_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ), are examples of perovskitematerials applicable to this invention. Most notable areLa_(1−x)Rb_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ) where 0.15<x<0.25, 0<y<0.2,1<z<1.02; La_(1−x)Cs_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ), where 0.1<x<0.3,0<y<0.2, 1<z<1.02; Pr_(1−x)Sr_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ) where0.3<x<0.5, 0<y<0.2, 1<z<1.02; Nd_(1−x)Ca_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ)where 0.3<x<0.5, 0<y<0.2, 1<z<1.02; andNd_(1−x)Sr_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ) where 0.3<x<0.5, 0<y<0.2,1<z<1.02.

Of these compositions, the most catalytic and stable isNd_(1−x)Ca_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ) where 0.3<x<0.5, 0<y<0.2,1<z<1.02. X-ray diffraction patterns showed no decomposition of highsurface area powder of this composition when exposed to 33% NaOH at 90 Cfor 1000 hours and cathodes were prepared which exhibited very low overpotential when operated at relatively high current density at 90 C in33% NaOH.

The carbon black used preferably with the present invention is in theform of fine particles, and for example, carbon black having a BETspecific surface area of 30 to 2,000 m²/g. That is, materials calledacetylene black, furnace black, channel black, thermal black, and thelike may be used. Those mentioned above may be used alone, or at leasttwo materials among those mentioned above, which have differentproperties, such as particle diameters and hydrophilic properties, fromeach other, may be effectively used in combination whenever necessary.For the gas diffusion layer, carbon particles having high waterrepellent properties are preferably used. The particle diameter of thecarbon black is preferably in the range of from 0.01 to 0.1 μm.

Method of Making Catalyst

Any method may be used to prepare the catalyst as long as a desiredoxide can be obtained. Two exemplary and suitable methods will bedisclosed below. In summary, a first method comprises mixing metaloxides together so as to obtain a mixture having a predetermined molarratio followed by calcining. A second method comprises the steps of:preparing an aqueous solution containing metal salts at a predeterminedmolar ratio with one or more chelating agents such as ethylene glycoland citric acid; heating to gel the solution then to form a char;followed by calcining.

However, a third method, according to one aspect of the invention willbe disclosed which is particularly suited for obtaining excellentdispersion of the catalyst with the carbon used in electrodefabrication. In this method a portion or none of the desired oxide ispremixed with the carbon black to ultimately be used for electrodefabrication. The balance of the desired oxide is prepared by adding anaqueous solution containing metal salts at a predetermined molar ratioalong with a dispersant. The slurry is dried then calcined in anon-oxidizing environment.

First Method of Preparing Catalyst

In the first method powdered metal oxides or powdered metal carbonatesare mixed together so as to form an oxide mixture having a desiredcomposition. Any mixing method may be used as long as the powdered metaloxides or the powdered metal carbonates are sufficiently mixed together.However, the mixing is preferably performed while the powdered materialsare pulverized and de-agglomerated using a mortar and pestle or a mill.The resultant mixture thus formed is calcined by heating, therebyforming the oxide. The oxide may be analyzed by X-ray diffraction toverify that the desired perovskite structure has been obtained.Following calcining, the oxide may be milled to decrease particle sizeand thereby increase the specific surface area.

Second Method of Preparing Catalyst

In the second method an aqueous solution is prepared containing metalsalts at a predetermined molar ratio with one or more agents such asethylene glycol and citric acid. This solution is heated to gel thesolution then to form a char; followed by calcining. The metal salts,can be for example, a chloride, a nitrate, a sulfate, a carbonate, or anacetate. Again, the oxide may be analyzed by X-ray diffraction to verifythat the desired perovskite structure has been obtained. Typically theoxide is attained at lower temperature in this method, compared to thefirst method described above. Following calcining, the oxide may bemilled to decrease particle size and thereby increase the specificsurface area. However, since the desired oxide can be obtained at alower calcining temperature, less milling is typically required toattain comparable surface area. Accordingly, this method is a preferablemethod for preparing a catalyst since fewer impurities are introducedduring milling.

Third Method of Preparing Catalyst

In this method, some or all of the perovskite oxide catalyst is preparedin the presence of the high surface area carbon black used in thereaction layer. A portion or none of the desired oxide is premixed withthe carbon black to ultimately be used for electrode fabrication, thenthe balance of the desired oxide is prepared similar to the secondmethod described above, with the exception that the fine carbon is addedto the aqueous metal salt solution and the material is heated in anon-oxidizing atmosphere such as nitrogen or carbon dioxide to preventoxidation of the carbon. With this method the perovskite oxide catalystis highly dispersed with the carbon and the electrode is very active.

Any heating condition for calcining, including a temperature and aholding time, may be used as long as a desired single phase oxide can beobtained. The optimum temperature is dependent on the actual material;however, the temperature is preferably in the range of from 500° C. to1,400° C. and is more preferably in the range of from 600° C. to 1,200°C. When the heating for calcining is performed at a higher temperature,the particle diameter is increased, and hence the surface area isdecreased. The holding time is preferably in the range of from 1 to 10hours. When the heating for calcining is performed together with carbon,in order to prevent the carbon from being oxidized, the heating ispreferably performed in a non-oxidizing atmosphere containing nitrogen,carbon dioxide or the like. However, when the desired oxide can beobtained at a low temperature so that the carbon may not be oxidized,the heating for calcining may be performed in air or in an atmospherecontaining oxygen.

Construction of Electrode

In the present invention, in order to form the gas diffusion electrodeby bonding the carbon black particles and the powdered oxide catalyst, afluorinated resin is used as a binder. As the fluorinated resin, forexample, polytetrafluoroethylene (PTFE), fluorinated ethylene propylenecopolymer (FEP), tetrafluoroethylene perfluoroalkoxy vinyl ethercopolymer (PFA), ethylene tetrafluoroethylene copolymer (ETFE), andpolychlorotrifluoroethylene (PCTFE) may be used. As the binder, apowdered fluorinated resin may be used. However, in order to improve thedispersibility of the carbon and the oxide catalyst, a dispersioncomposed of fine fluorinated resin particles dispersed in water by asurfactant is preferably used.

FIG. 1, discloses a gas diffusion electrode 10 having a multilayerstructure composed of a gas diffusion layer 12, a reaction layer 14, anda current collector 16. The current collector 16 is provided inside theelectrode 10 for electrical connection. Oxygen (indicated by the arrowlabeled “O”) is supplied from the gas diffusion layer side 15. Thereaction layer 14 is in contact with an electrolyte 20. After passingthrough the gas diffusion layer 12, the oxygen “O” is reduced on anoxygen reduction catalyst 18 as described above, which is fixed in thereaction layer 14.

The reaction layer 14 is preferably formed as follows. The powderedcatalyst prepared by the methods described above, are dispersed in asolvent along with carbon as described above. A dispersion containing afluorinated resin is added to the resultant solvent described above,thereby forming a new dispersion. As a dispersion method, any method maybe used as long as the catalyst, the carbon, and the fine fluorinatedresin particles can be highly uniformly dispersed. Preferably a methodusing ultrasonic wave is used. As the solvent, water is primarily used,but an alcohol such as ethyl alcohol and other organic solvent may alsobe used. In addition, in order to improve the dispersibility, of course,various surfactants may also be used.

When the dispersion thus obtained is filtrated, washed, and dried, apowder for the reaction layer is obtained. Any drying method may be usedas long as the solvent can be removed. How well the powder is dried canaffect how effectively it can be distributed evenly for electrodefabrication. The amount of the oxide catalyst with respect to that ofthe carbon black is preferably in the range of from 10 to 90 percent byweight. When the amount of the oxide catalyst is too small, the totalreaction surface area of the catalyst is decreased, and as a result, asufficient oxygen reduction activity may not be obtained, resulting indegradation of the properties of the gas diffusion electrode. On theother hand, when the amount of the catalyst is too large, the catalystis liable to agglomerate, and as a result, the total reaction surfacearea is decreased, or the electric conductivity may be decreased in somecases, resulting in degradation of the properties of the gas diffusionelectrode.

In addition, the amount of the fluorinated resin is preferably in therange of from 5 to 30 percent by weight with respect to the total amountof the carbon black and the oxide catalyst. When the amount of thefluorinated resin is too small, the bonding force is low, and as aresult, the strength of the gas diffusion electrode is not satisfactory.On the other hand, when the amount of the fluorinated resin is toolarge, the reaction surface area of the catalyst is decreased, and, theelectric conductivity is also decreased, resulting in significantdegradation of the properties of the gas diffusion electrode.

A powder for the gas diffusion layer can be formed by the same method asthat for the reaction layer except that the catalyst is not used.

The powder for the reaction layer and the powder for the gas diffusionelectrode are placed in a mold together with the current collector, andmolding is then performed by hot pressing, thereby forming the gasdiffusion electrode. Alternatively, a large gas diffusion electrode maybe formed by the steps of adding solvents to the respective powdersdescribed above to form pastes, forming films from the individual pastesmentioned above, laminating the above-mentioned films together with thecurrent collector, and performing hot pressing. The current collectormay be formed of any material as long as it has sufficient electricconductivity for electrical connection and is not dissolved nor corrodedat a potential at which electrochemical oxygen reduction occurs. Variouswire meshes and foam metals may be used. For example, a silver or nickelmesh of #30 to #150 or foam nickel plated with silver may be preferablyused.

In hot pressing, in order to obtain superior strength and durability ofthe electrode and, to form pores therein for smoothly supplying oxygento the catalyst, the molding condition for bonding the carbon and theelectrode catalyst to each other with the fluorinated resin binder mustbe selected. The press temperature is preferably in the range of from350 to 390° C. which is close to the melting point of a fluorinatedresin, and the molding pressure is preferably in the range of from 30 to150 kgf/cm².

The total thickness of the electrode can be formed in the range of from0.5 to 1.2 mm, and the thickness of the reaction layer and the thicknessof the gas diffusion layer are preferably 0.05 to 0.3 mm and 0.45 to 0.9mm, respectively. As long as the strength, the durability, and theelectrode performance are all satisfied, the thicknesses are preferablyas small as possible.

After the gas diffusion electrode formed as described above is placed ina cell for evaluation of electrochemical characteristics, oxygen or airis supplied from the gas diffusion layer side so that electrochemicaloxygen reduction occurs, and the electrode potential are measured atvarious current densities, to evaluate the electrode performance.

The following Examples will further illustrate the advantages, aspects,and enablement of the present invention.

EXAMPLE 1 Preparation of Oxide Catalyst

First, an aqueous solution of neodymium nitrate, an aqueous solution ofcalcium nitrate, an aqueous solution of manganese nitrate and an aqueoussolution of iron nitrate were mixed together in the appropriate molarproportions to form a mixed aqueous nitrate solution for makingNd_(0.6)Ca_(0.4)(Mn_(0.8)Fe_(0.2))_(1.01)O_(3+δ). Ethylene glycol andcitric acid were added as chelating agents. This mixed aqueous solutionwas then heated in a glass container until the solution boiled andeventually ignited in the presence of air. Following ignition and anadditional rise in temperature to about 350 C the remaining oxideprecursor self-dried. Following drying, the remaining contents whereplaced in a conditioned sagger and calcined in air at 800 C for 8 hours.After calcining, X-ray diffraction analysis indicated the material tohave a single phase of the perovskite structure. After the calcining thematerial was ball milled for 24 hours to attain a surface area of 17.2m²/g according to BET analysis.

Stability Test of Catalyst Material

To evaluate the chemical stability of the oxide catalyst, high surfacearea samples were immersed in an aqueous solution of sodium hydroxide ata concentration of 33 percent by weight and at a temperature of 90° C.for 200, 500, and 1,000 hours. The XRD pattern of the oxide powdermeasured at each time mentioned above and that measured before immersionwere observed. The results are shown in FIG. 2. Unlike most of the oxidecatalyst candidates, Nd_(0.6)Ca_(0.4)(Mn_(0.8)Fe_(0.2))_(1.01)O_(3+δ),remained a single phase perovskite oxide, and did not decompose in partto a hydroxide of one or more of the material's constituents. The XRDpattern of the electrode, which was immersed in the aqueous solution ofsodium hydroxide for 1,000 hours, was not substantially changed, andhence it was confirmed that the catalyst is stable even in an aqueoussolution of sodium hydroxide at a high temperature.

Preparation of Powder for Reaction Layer

A reaction layer mixture containing the perovskite synthesized above wasprepared by mixing 36 parts perovskite, 25 parts carbon black, forexample KETJEN BLACK EC600JD (BET specific surface area of 1,270 m²/g)from Lion Corporation, and 11 parts of a second carbon black, forexample DENKA BLACK AB-7 from Denki Kagaku Kogyo K.K. Thus the catalystto carbon ratio was 1:1. Then 10000 parts hot water (about 88 C) wasadded to the perovskite/carbon mixture and mixed with a high shear mixerfor 2.5 minutes. Then 1000 parts ethanol was slowly added to the mixtureduring high shear mixing. These contents were transferred to a lowenergy mixer. In a separate vessel, 24 parts of apolytetrafluoroethylene dispersion, for example POLYFLON TFE D-1 (solidcomponent of 60 percent by weight) from Daikin Industries, Ltd., wasmixed with 5000 parts hot water. That mixture was added to the contentsalready in the low energy mixer, and the resultant dispersion was thenstirred. Next, 24000 parts of ethyl alcohol was slowly added to thisresultant dispersion, followed by more low energy mixing then filtrationthrough 1 μm pore size filter media. The filter cake was initially airdried at 40 C then subsequently dried at 150 C for 4 hours, thenpulverized using a mill to form fine powder for the reaction layer.

Preparation of Powder for Gas Diffusion Layer

40 parts carbon black, for example DENKA BLACKAB-7 from Denki KagakuKogyo K.K., and 1600 parts hot water were mixed in a high shear mixerfor 5 minutes. Next 28 parts polytetrafluoroethylene dispersion, forexample POLYFLON TFE D-1 (solid component of 60 percent by weight) fromDaikin Industries, Ltd., and 500 parts hot water were combined thenadded to the carbon/water mixture with low energy mixing for 5 minutes.Next 800 parts ethanol was slowly added while mixing with a low energymixer.

Next the resultant suspension thus obtained was filtered through 1 μmpore size filter media. The filter cake was initially air dried at 40 Cthe subsequently dried at 150 C for 4 hours, then pulverized using amill to form fine powder for the gas diffusion layer.

Formation of Gas Diffusion Electrode

After an aluminum foil degreased with acetone was placed at the bottomof a hot press mold having an inside diameter of 2.54 cm, 0.1 g of thepowder for the reaction layer was evenly distributed in the die atop thefoil, then 0.2 g of the gas diffusion layer powder was evenlydistributed atop the reaction layer powder. The layers wereprecompressed with light pressure, then a nickel mesh of #100 having awire diameter of 0.1 mm was placed on the precompressed powders. Afterthe composite thus prepared was heated to 380° C., hot pressing wasperformed at 134 kgf/cm² for 1 minute, thereby obtaining the gasdiffusion electrode.

Measurement of Polarization

Evaluation of electrochemical characteristics of electrochemical oxygenreduction was performed, in which the gas diffusion electrode was placedin a cell for evaluation of electrochemical characteristics and oxygengas was supplied at a rate of about 50 standard ml/min from the gasdiffusion layer side into an aqueous solution of sodium hydroxide at aconcentration of 33 percent by weight and at a temperature of 90° C.

In the cell for evaluation of electrochemical characteristics, flow pathwas provided each for oxygen supply and discharge, and the gas diffusionelectrode was fixed to the cell using an o-ring, so that the inside ofthe cell was gas sealed. The cell thus prepared was fitted to anelectrolytic bath, and the evaluation of electrochemical characteristicswas performed. In this evaluation, the reaction layer side was only incontact with the aqueous solution of sodium hydroxide at a concentrationof 33 percent by weight and at a temperature of 90° C., and oxygen wassupplied from the gas diffusion layer side. The effective area of thegas diffusion electrode was 2.0 cm². As a DC stabilized power supply wasused, a nickel mesh electrode was used as a counter electrode, and amercury/mercury oxide electrode was used as a reference electrode. FIG.3 shows the polarization curve of a cathode made with flowing oxygenmade as described above. FIG. 4 is a plot of cathode over-potentialversus current density assuming a standard potential of 0.401 V for thereaction O₂+2H₂O+4e⁻=4OH⁻, and standard potential of 0.097V for thereference electrode (HgO+H₂O+2e⁻=Hg+2OH⁻). This data shown that thiselectrocatalyst is very active for oxygen reduction.

EXAMPLE 2

The gas diffusion electrode was formed in the same manner as that inExample 1 except that Nd_(0.6)Ca_(0.4)Mn_(1.01)O_(3+δ) was the catalystoxide synthesized instead ofNd_(0.6)Ca_(0.4)(Mn_(0.8)Fe_(0.2))_(1.01)O_(3+δ). The chemical stabilityof the oxide catalyst was again evaluated where high surface areasamples were immersed in an aqueous solution of sodium hydroxide at aconcentration of 33 percent by weight and at a temperature of 90° C. for1,000 hours. After 1000 hours time the XRD analysis indicated that only0.16% of the material had reacted to form a hydroxide second phase basedon comparison of the peak areas. FIG. 5 shows the diffractogramsindicating the material to be very stable as in Example 1. Some of thematerial from this example was milled in an attrition mill for 16 hoursto reach a specific surface area of 30.3 m²/g. Subsequently cathodeswere made and tested electrochemically as in Example 1. FIG. 3 shows thepolarization curves of cathodes with flowing oxygen. FIG. 4 is a plot ofcathode over-potential versus current density for the cathodes. Thepolarization curve is very similar to that of the electrode prepared inExample 1. FIG. 6 shows the individual performance of a second electrodemade according to Example 2. The polarization curve shows behavior after16 hours and 200 hours of operation at 0.4 A/cm², 90 C, and 33% NaOH.Both curves are nearly the same showing that the electrode catalyst isvery stable as well as active. FIG. 7 shows the same data expressed interms of over-potential versus current density.

EXAMPLE 3

To further increase the activity of the catalyst a portion of thecatalyst was made in the presence of the carbon to be used for thereaction layer. Nd_(00.6)Ca_(0.4)Mn_(1.01)O_(3+δ), prepared in example 2was mixed with carbon black in the proportions of 8 parts perovskite, 7parts carbon black, for example KETJEN BLACK EC600JD (BET specificsurface area of 1,270 m²/g) from Lion Corporation and 3 parts of asecond carbon black, for example DENKA BLACK AB-7 from Denki KagakuKogyo K.K. Thus the catalyst to carbon ratio was 4:5 at this point. Next80 parts water was added and 0.3 parts oleic acid to serve as adispersant.

Next, an aqueous solution of neodymium nitrate, an aqueous solution ofcalcium nitrate, an aqueous solution of manganese nitrate were mixedtogether in the appropriate molar proportions to form a mixed aqueousnitrate solution for making Nd_(00.6)Ca_(0.4)Mn_(1.01)O_(3+δ) ofsufficient quantity such that the final ratio of perovskite oxide tocarbon will be 1:1 after adding to the pre mixed perovskite and carbon.After low energy mixing, the material was dried at 80 C. After drying,the material was calcined in a nitrogen gas environment at 700 C for 1hour then cooled.

This material was made into a reaction layer material as described inExample 1 except no additional carbon was added. Next a gas diffusionelectrode was made as described in Example 1 with this material. Thecathodes were tested in a cell with a nickel mesh anode and Hg/HgOreference electrode, with 33% NaOH at 90 C. FIG. 3 shows thepolarization curve of the cathode with flowing oxygen. FIG. 4 is a plotof the cathode over-potential versus current density. While the catalystcomposition and ratio of catalyst to other constituents in this exampleare the same as example 2, cathodes made in this manner are much moreactive and as such have lower over potential.

FIG. 8 shows a plot of the polarization curves of Examples 1, 2, and 3against data reported in the literature for cathodes made from othermaterials. All of the examples presently disclosed performed best athigher current densities.

EXAMPLE 4

To determine the effect of whether a catalyst is “A” site rich or “B”site rich, two materials were made as in example 1, except onecomposition, La_(0.8)Rb_(0.2)Mn_(0.99)O_(3+δ), was A site rich while theother composition, La_(0.8)Rb_(0.2)Mn_(1.01)O_(3+δ), was B site rich.Gas diffusion electrodes with each material were made as described inExample 1 and they were tested under the same cell conditions as inExample 1. The respective polarization curves after 16 hour and 100 houroperation at 0.4 A/cm², 90 C and 33% NaOH of the 2 materials are shownin FIG. 9. The data shown in FIG. 9 shows that the “B” site richcomposition had a more favorable polarization curve, with higherpotentials than the “A” site rich composition. Also the XRD pattern ofthe respective electrodes after 100 hour operation showed less secondphase formation in the “B” site rich composition compared to the “A”site rich composition.

COMPARATIVE EXAMPLE 1 Formation of Silver Supported Carbon

First, 1.0 g of carbon black, for example KETJEN BLACK EC600JD (BETspecific surface area of 1,270 m²/g) from Lion Corporation, was added to50 ml of an aqueous solution of silver nitrate at a concentration of0.184 mol/l and was sufficiently stirred so as to have a uniformmixture. Subsequently, after the mixture thus obtained was dried byevaporation at 100° C. for 24 hours, heating was performed at 300° C.for 1 hour in a nitrogen atmosphere, and as a result, silver-supportedcarbon at a ratio of silver to carbon of 1 to 1 on a weight basis wasformed.

Preparation of Powder for Reaction Layer

Next, 0.3 g of the silver-supported carbon thus formed was dispersed in30 ml of water, and this mixture was then added to 0.1 g of apolytetrafluoroethylene dispersion POLYFLON TFE D-1 (solid component of60 percent by weight) from Daikin Industries, Ltd., followed bysufficient stirring. Next, after 50 g of ethyl alcohol was added to thismixture and was then stirred, filtration was performed, and the solidcomponent thus obtained was then dried at 100° C. for 24 hours.Subsequently, the resultant solid product was pulverized using a millinto fine particles, thereby obtaining the powder for reaction layer.Next, the powder thus obtained was washed with ethyl alcohol and water.

Hereinafter, the gas diffusion electrodes were formed in the same manneras that in Example 1 except that the powder for reaction layercontaining silver described above was used. Furthermore, in the samemanner as that in Example 1, the evaluation of electrochemicalcharacteristics was performed.

The results of the evaluation of electrochemical characteristicsconfirmed that the electrode using Nd_(0.6)Ca_(0.4)(Mn_(0.8)Fe_(0.2))_(1.01)O_(3+δ) or Nd_(0.6)Ca_(0.4)Mn_(1.01)O_(3+δ) asa catalyst has a potential higher than that of the electrode usingsilver at a current density of 0.4 A/cm² and has superiorcharacteristics.

1. A method for diffusing a catalyst with carbon for use in a gasdiffusion electrode reaction layer, the method comprising the steps of:preparing an aqueous solution comprising at least one metal salt at apredetermined molar ratio and carbon; heating the aqueous solution in anon-oxidizing atmosphere to a gel and further heating said gel to form achar; and calcining the char.
 2. The method of claim 1, including thestep of mixing a quantity of the desired oxide catalyst with the carbonbefore adding the carbon to the aqueous solution, the amount of premixedmetal oxide being chosen in conjunction with the amount of said at leastone metal salt to provide the desired molar ratio after calcining. 3.The method of claim 1, wherein the carbon is in the form of fineparticle carbon black having a BET specific surface area of 30 to 2,000m²/g.
 4. The method of 1, wherein the catalyst comprises the formulaA_(1−x)A′_(x)(B_(1−y)B′_(y))_(z)O_(3+δ), wherein A comprises at leastone metal chosen from La, Pr and Nd; wherein A′ comprises at least onemetal chosen from K, Rb, Cs, Ca, Sr and Ba; wherein B comprises at leastone metal chosen from Mn and Co; and wherein B′ comprises at least onemetal chosen from Fe and Ni.
 5. The method of claim 1, wherein thecarbon is in the form of fine particle carbon black having a BETspecific surface area of 500 to 1500 m²/g.
 6. The method of claim 1,wherein the resulting catalyst comprises the formulaNd_(1−x)Ca_(x)(Mn_(1−y)Fe_(y))_(z)O_(3+δ), wherein x has a value in therange of about 0.01 to about 0.90; y has a value in the range of about0.0 to about 0.90; δ has a value in the range of about −0.3 to about0.30 and z has a value in the range of about 0.95 to about 1.05.
 7. Themethod of claim 6, wherein x has a value in the range of about 0.1 toabout 0.6; y has a value in the range of about 0.0 to about 0.4; 6 has avalue in the range of about −0.3 to about 0.3 and z has a value in therange of about 0.95 to about 1.05.
 8. The method of claim 6, wherein xhas a value in the range of about 0.2 to about 0.5.
 9. The method ofclaim 6, wherein y has a value in the range of about 0.0 to about 0.3.10. The method of claim 1, wherein the carbon is from acetylene black,furnace black, channel black, and thermal black.
 11. The method of claim2, wherein the carbon is chosen from acetylene black, furnace black,channel black, and thermal black.
 12. The method of claim 4, wherein thecarbon is chosen from acetylene black, furnace black, channel black, andthermal black.